Traditionally, there is proposed an air conditioner for a vehicle which performs dehumidification heating operation by utilizing a heat pump cycle (see Japanese Patent Application Laid-Open Publication No. 2013-256230). For example, the air conditioner 91 for the vehicle as shown in FIG. 13 is provided with a compressor 21, an indoor heat exchanger 22, an expansion valve A 23, an outdoor heat exchanger 24, an expansion valve B 25 and an evaporator 26 as component elements. In the dehumidification heating operation, a refrigerant is circulated in the order named in a circuit composed of these component elements. The evaporator 26 and the indoor heat exchanger 22 are accommodated in a duct 51, and the evaporator 26 is arranged upstream of the indoor heat exchanger 22 relative to a circulating direction of air-conditioning air within the duct 51.
As a method of regulating a temperature of the air-conditioning air, there is known a method of adjusting an opening ratio between the expansion valve A 23 and the expansion valve B 25 so as to lower the temperature of the evaporator 26 to zero or below, and of raising the indoor heat exchanger 22 provided downstream of the evaporator 26 relative to the circulating direction of the air-conditioning air to a target temperature or above.
In this method, an opening degree of the expansion valve B 25 between the evaporator 26 and the outdoor heat exchanger 24 arranged upstream of the evaporator relative to the circulating direction of the refrigerant is controlled so as to generate a pressure difference between the refrigerant of the evaporator 26 and the refrigerant of the outdoor heat exchanger 24. When the outside air temperature is low, the temperature of the evaporator 26 is required to be higher than an evaporator freezing limit temperature (for example, zero degrees centigrade) so as to prevent freezing. Further, the temperature of the outdoor heat exchanger 24 installed on the upstream side of the evaporator 26 relative to the circulating direction of the refrigerant is not able to be lowered below the temperature of the evaporator 26. A temperature difference between the temperature of the refrigerant of the outdoor heat exchanger 24 and an outside air temperature is not able to be increased, so that a heat absorption amount from the outside air supplied to the outdoor heat exchanger 24 to the refrigerant is not obtained sufficiently. Therefore, there may be cases where a sufficiently high temperature as a desired blow-off temperature is not obtained. The blow-off temperature means the temperature of the air-conditioning air discharged from the duct 51. Generally, an upper limit of controllable blow-off temperature tends to become lower as the outside air temperature becomes low.
Therefore, like an air conditioner 92 for a vehicle as illustrated in FIGS. 14A and 14B, such a configuration that the outdoor heat exchanger 24 and the evaporator 26 are arranged in parallel to each other in the circulation circuit of the refrigerant and a throttle valve 38 is provided in an exhaust port of the refrigerant from the evaporator 26 is proposed. According to this configuration, an evaporation temperature of the refrigerant of the evaporator 26 can be raised above the temperature of the refrigerant of the outdoor heat exchanger 24 by controlling an opening degree of the throttle valve 38. Thus, it is possible to achieve both ensuring of the heat absorption amount in the outdoor heat exchanger 24 and prevention of the freezing of the evaporator 26.